9. Rule number 3

By now, you should be ready to learn about the third rule for writing computer programs efficiently.

Important

Rule # 3

When writing computer programs, do not repeat yourself. I repeat: do not repeat yourself!

9.1. Three lefts can make a right

Since you have written quite a few programs by now (you have done the exercises, didn’t you?), you must have realized that having Reeborg make three left turns in a row gives the same final result as if he were to make a single right turn. You are probably starting to find it quite tedious to have to write three turn_left() instructions each time you want to have Reeborg turn right. Wouldn’t it be nicer if you could simply write turn_right()?

By the end of this lesson, you will be able to do just that.

9.2. Defining functions

What we have called instructions or commands so far are examples of Python functions. So, turn_left() is a function and so is move(). We can define a new Python function as follows:

def some_well_chosen_name ():
    # some lines of code
    # indented at the same level

def is our first Python keyword. Keywords are words that have a special meaning in a given programming language. Notice how a colon : precedes what is known as a block of code and constitutes the body of the function. It is required in Python to indent such blocks of code with the same number of spaces at the beginning of each line which, as a side-effect, makes it easy to identify the function body for a human reader.

Let’s write our first Python function:

def turn_right():
    turn_left()
    turn_left()
    turn_left()

That’s it! You will now be able to avoid having to write three turn_left() functions in a row to simulate a right turn!

Try it!

Define the turn_right() function in a program and try to use it, perhaps using the world Alone. If you don’t know what to try, just have Reeborg go around a square by alternating turn_right() and move() functions four times in a row.

9.3. How to think about def

Note

Please note that this simplified explanation does not take into account what is known as variable scope which is something that we will cover later.

You have just seen how to define a function in Python. Chances are, you understood right away how to think about them but, just in case, here’s a more detailed explanation which will help you to not only understand how functions work, but also other Python construct that involve blocks of code.

Suppose we have the following:

 def turn_right():  # begin of code block follows
     turn_left()
     turn_left()
     turn_left()   # end of code block

 move()
 turn_right()
 move()

This is equivalent to the following:

 # define a function
 def turn_right():
     turn_left()
     turn_left()
     turn_left()

 move()
 # begin of code block inside turn_right()
 turn_left()
 turn_left()
 turn_left()
 # end of code block
 move()

In other words, def defines a name that we can use as a synonym for all the code that appears inside the code block, and whenever we see the synonym being called [that is, the name appears followed by ()], we can think of it as being equivalent to inserting the code block as is at that location.

Try this!

See if you can simplify your program for the newspaper delivery and making it easier to read and understand by using a turn_right() function. Once you have done so, perhaps you can think of other ways in which you can simplify your program by defining other functions.